The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device using silicon carbide.
Because of their excellent properties, field effect transistors using silicon carbide are anticipated to be used in a high temperature environment at a temperature of, for example, at least 150° C., preferably at a temperature of at least 300° C. which cannot be achieved with field effect transistors using silicon.
However, field effect transistors using silicon carbide cannot be applied to any high temperature environment. Since there is an upper limit in their operation temperature, a temperature sensor must be mounted. In field effect transistors using silicon, the temperature is sensed by using a diode formed of polycrystalline silicon on the same substrate as a transistor via an insulation film as disclosed in, for example, “Power Device, Power IC Handbook,” first printing in first edition, published by CORONA PUBLISHING CO., LTD. on Jul. 30, 1996, (FIG. 7.53).
However, the diode formed of polycrystalline silicon does not operate in a high temperature environment anticipated in the case of silicon carbide, and it cannot be used as the temperature sensor. Therefore, it is necessary to sense the high temperature by using a diode using silicon carbide or a hetero junction diode formed of silicon carbide and polycrystalline silicon.
If a diode such as a p-n junction diode, a hetero junction diode or a Schottky diode can be formed on the same substance as a field effect transistor together with the field effect transistor for the purpose of not only sensing at high temperatures but also overcurrent sensing or overvoltage sensing, a great benefit can be obtained from the viewpoint of cost as well.
As for field effect transistors, there are normally-off type JFETs (Junction Field Effect Transistors) having a trench as disclosed in JP-A-9-508492 (FIGS. 6 to 11), Materials Science Forum, Vols. 433-436 (2003), pp. 777-780, and IEEE Electron Device Letters, Vol. 24, NO. 7, pp. 463-465. In these JFETs, normally-off type is implemented by determining the concentration and width of control layers so as to bring the control layers (control regions) that are formed along trenches and that are different in conductivity type from a substrate into a pinched-off state when a voltage is not applied.